1. Field of the Invention
The present invention relates to an ultrasonic motor.
2. Description of Related Art
Various Langevin type ultrasonic motors, each of which has a generally cylindrical shape and generates a standing wave, have been known. Such an ultrasonic motor is disclosed in, for example, U.S. Pat. No. 6,018,213 (corresponding to Japanese Unexamined Patent Publication No. 11-155288), the contents of which are incorporated herein by reference. The disclosed ultrasonic motor includes a cylindrical stator and a cylindrical rotor. The stator includes a plurality of metal blocks and two piezoelectric elements held between corresponding two of the metal blocks. These metal blocks and the piezoelectric elements are held together by a bolt inserted through the metal blocks and the piezoelectric elements in an axial direction of the stator. One of the metal blocks below the piezoelectric elements has slits, which are arranged in an outer peripheral surface of the metal block at equal intervals to convert a longitudinal vibration generated by the piezoelectric elements into a torsional vibration. The rotor is slidably and rotatably urged against a top end surface of the stator, i.e., a top end surface of the uppermost metal block by an urging mechanism.
In the disclosed ultrasonic motor, when a high frequency voltage is applied to the piezoelectric elements, a longitudinal vibration is generated by the piezoelectric elements, and a corresponding torsional vibration is generated by the slits in response to the longitudinal vibration. With these vibrations, the rotor is rotated relative to the stator.
In the above disclosed ultrasonic motor, an annular flange-like securing projection is formed in the lowermost metal block located below the above-described slits to secure the stator and the rotor to an external member, such as a motor case. When the ultrasonic motor is driven, the above described securing projection tends to vibrate substantially. Specifically, in an experiment, when a lower end (a lower end surface) of the stator (the lowermost metal block) is placed over a flat plate, and the ultrasonic motor is driven in this state, the securing projection tends to vibrate substantially (more specifically, a lower end surface of the securing projection vibrates at a relatively high vibration velocity). When the securing projection is secured to the external member, such as the motor case, and the ultrasonic motor is driven, a large vibration is conducted from the securing projection to the external member, such as the motor case, and/or the longitudinal vibration and the torsional vibration of the stator may be adversely affected by the vibration of the securing projection. This will cause deterioration of the performance (e.g., causing fluctuations in a resistivity or a rotational speed) of the ultrasonic motor.